Cavity optomechanics: towards sensing at the quantum limit

试图将一个小型机械设备冷却到量子基态的巨大挑战正在全球许多领先的实验小组中推动着激烈的活动。十年前似乎不可能实现的目标，现在看起来非常接近了：通过光力学技术，像小镜子和悬臂这样的微机械谐振器已经被冷却了几个数量级，达到了n~30个数量级。接近基态（n~1）的最终目标现在看来是一个现实的前景，尽管仍然存在严重的障碍；其中，对环境的热耦合最为严重。然而，在去年，三个小组（包括PI的小组）独立地提出了一个新颖的方案，该方案具有基本的新设计：一个介电纳米球，光学悬浮在一个腔中，由光场产生的偶极子力冷却。在某种意义上，缺乏与腔体结构的机械连接将设备与重要的热噪声源隔离开来，并使该方案与传统设备相比具有独特的优势。该项目汇集了来自伦敦和南安普顿的实验和理论小组，最终目标是首次成功地在实验中实施这一方案。此外，我们的目标是通过对光学冷却纳米球系统进行完整和现实的模拟，从理论上彻底理解其潜在的物理原理。一旦达到量子极限，主要目标就是在这个范围内操作设备。潜在的回报是巨大的。这是一种可实现的量子技术，它提供了无与伦比的测量灵敏度的前景，仅受海森堡测不准原理的限制。例如，正是由于这个原因，这些设备被用于引力波探测器，这需要非常精确地检测位移。它们还提供了对量子经典边界的基本见解的可能性：有可能研究仅因宏观物体的位移而不同的叠加态。一些实验小组正在研究用玻色-爱因斯坦凝聚体作为机械振荡器的偶极-力耦合腔光力学。在这种情况下，目标已经处于基态，因此已经有可能探索量子状态。我们还将从理论上研究这一机制，以确定在这样的方案中是否可能产生像压缩（在量子机制中改善传感）这样的量子效应，因为两个共同申请者已经确定了一个潜在的有前途的机制。最后，从长远来看，我们注意到，在这项工作的同时，小型传感器，如微米级悬臂正在积极开发用于生物传感应用（用于超灵敏的生物分子检测或作为力传感器）。伦敦大学学院，特别是LCN（伦敦纳米技术中心）是这一领域的领导者。另一方面，致力于将光机械设备冷却到量子极限的团队（如加州理工学院的Vahala团队）已经在测试它们作为生物传感器的潜力。从长远来看，一个理想的目标是实现这两个方向的合并：量子有限探测和生物传感。我们将探索采用基于我们的介电纳米球的方案的可行性。

The grand challenge of attempting to cool a small mechanical device towards its quantum ground state is driving intense activity in many leading experimental groups worldwide. What seemed an unfeasible target only a decade ago, now appears tantalisingly close: by means of optomechanical techniques, micromechanical resonators such as small mirrors and cantilevers have been cooled by several orders of magnitude, down to occupation numbers of order n~30. The ultimate goal of approaching the ground state (n~1) now seems a realistic prospect, although serious obstacles remain; among these, thermal coupling to the environment is the most serious.However, within the last year, three groups (including the PI's) have independently proposed a novel scheme which has a fundamental new design: a dielectric nanosphere, optically levitated in a cavity and cooled by dipole forces arising from the optical field. The lack of mechanical connection to the cavity structure in a sense insulates the device from important sources of thermal noise and gives this scheme a unique edge in relation to conventional devices. The project brings together experimental and theory groups from London and Southampton with the ultimate goal of successfully implementing this scheme experimentally, for the first time. In addition, we aim to thoroughly understand the underlying physics theoretically by undertaking complete and realistic simulations of the optically cooled nanosphere system.Once the quantum limit is achieved, the main target is to operate the device in this regime. The rewards are potentially great. This is an attainable quantum technology which offers the prospect of unparalleled sensitivity in measurement, limited only by the Heisenberg uncertainty principle. For example, it is for this reason that these devices are used for gravitational-wave detectors, which require extraordinarily precise detections of displacement. They offer also the possibility of fundamental insights into the quantum-classical border: it may be possible to investigate superpositions which differ only by the displacement of a macroscopic object. Some experimental groups are investigating dipole-force coupling cavity optomechanics using a BEC (Bose Einstein Condensate) as the mechanical oscillator. In this case, the target is already in the ground state so it is already possible to explore the quantum regime. We will also investigate this regime theoretically, in order to establish whether quantum effects like squeezing (which improve sensing in the quantum regime) may be viably generated in such a scheme, as two of the co-applicants have already identified a potentially promising regime.Finally, taking the long view, we note that in parallel to this work, small sensors such as micron-sized cantilevers are actively being developed for biosensing applications (for ultra-sensitive detection of biomolecules or as force sensors). UCL, in particular the LCN (London Centre for Nanotechnology) is a leader in this field. On the otherhand, groups (such as the Caltech group of Vahala) working to cool optomechanical devices to the quantum limit are already testing their potential as biosensors.A desirable ambition, in the long-term would be to achieve a merger of these two directions: quantum limited detection and biosensing. We will explore the viability of employing schemes based on our dielectric nanospheres.